krit.club logo

Cell - Cell Theory - The Unifying Principle of Biology

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

The Cell is the fundamental structural and functional unit of all living organisms. The term 'cell' was first coined by Robert Hooke in 16651665 while observing a thin slice of cork under a self-designed microscope.

•

Discovery Timeline: Antonie van Leeuwenhoek (16741674) discovered free-living cells in pond water; Robert Brown (18311831) discovered the nucleus; Purkinje (18391839) coined the term 'protoplasm' for the fluid substance of the cell.

•

The Cell Theory was proposed by Matthias Schleiden (18381838) and Theodore Schwann (18391839). It states: (1) All plants and animals are composed of cells, and (2) The cell is the basic unit of life.

•

Rudolf Virchow (18551855) expanded the cell theory by suggesting that all cells arise from pre-existing cells (Omnis cellula e cellulaOmnis\ cellula\ e\ cellula).

•

Organisms can be Unicellular (e.g., AmoebaAmoeba, ChlamydomonasChlamydomonas, ParamoeciumParamoecium, and bacteria) or Multicellular (e.g., fungi, plants, and animals).

•

Cells vary in shape and size depending on their function. For example, Nerve cells (neurons) are long to conduct impulses, while Human RBCs are circular and biconcave to transport oxygen.

•

Cell size is typically measured in micrometers (μm\mu m). 1 μm=10−6 m1\ \mu m = 10^{-6}\ m.

📐Formulae

Magnification=Size of ImageSize of Object\text{Magnification} = \frac{\text{Size of Image}}{\text{Size of Object}}

1 μm=10−6 m=10−3 mm1\ \mu m = 10^{-6}\ m = 10^{-3}\ mm

1 A˚ (Angstrom)=10−10 m1\ \text{\AA} \text{ (Angstrom)} = 10^{-10}\ m

Surface Area to Volume Ratio=SAV\text{Surface Area to Volume Ratio} = \frac{SA}{V}

💡Examples

Problem 1:

A cell is observed under a microscope with a magnification of 400×400\times. If the image of the cell appears to be 2 mm2\ mm in diameter, what is the actual size of the cell in micrometers (μm\mu m)?

Solution:

Given: Magnification (MM) = 400×400\times Image Size = 2 mm=2000 μm2\ mm = 2000\ \mu m Actual Size (AA) = ?

Using the formula: M=Image SizeActual SizeM = \frac{\text{Image Size}}{\text{Actual Size}} 400=2000 μmA400 = \frac{2000\ \mu m}{A} A=2000400 μmA = \frac{2000}{400}\ \mu m A=5 μmA = 5\ \mu m

Explanation:

The actual size of the cell is found by dividing the observed image size by the magnification power of the microscope. Since the final answer is required in micrometers, we converted 2 mm2\ mm to 2000 μm2000\ \mu m first.

Problem 2:

Who expanded the cell theory and what was the specific postulate added?

Solution:

Rudolf Virchow in 18551855 expanded the cell theory. The postulate added was: 'All cells arise from pre-existing cells' (Omnis cellula e cellulaOmnis\ cellula\ e\ cellula).

Explanation:

Schleiden and Schwann established that all living things are made of cells, but they did not explain how new cells are formed. Virchow's contribution completed the theory by explaining cell division as the source of new cells.